1. Field of the Invention
The present invention relates in general to methods for obtaining particulate calcium carbonate, and more particularly to methods for obtaining particulate calcium carbonate having uniformity of size such that when the particles are formed in the causticization process in a kraft pulp mill, the time required to separate the particles from liquors in which they are suspended is minimized and the amount of liquor recovered is maximized, with minimal dilution by water used for washing. Such uniformity of size also tends to improve the brightness and color of the particulate calcium carbonate.
2. Background of the Art
The kraft process, also known as the sulfate process, for extracting cellulose pulp from wood is practiced throughout the world. In many mills, the process is done in a sequential, closed loop that includes recovery of the spent chemicals. There are three (3) main operations in kraft pulping, and these are shown in FIG. 1.                a. Pulping, in which wood chips are cooked in a chemical solution, called white liquor, that consists primarily of caustic soda (NaOH) and sodium sulfide (Na2S).        b. Black Liquor Evaporation & Combustion, in which water is first removed from the spent cooking liquor followed by combustion to produce energy and recover non-combustible inorganic chemicals. The chemicals are recovered in a molten state, called a smelt, and dissolved in a dilute aqueous solution called weak wash that is produced in another part of the process. The dissolved smelt is called green liquor. The primary species in green liquor are sodium carbonate (Na2CO3), sodium sulfide (Na2S) and sodium hydroxide (NaOH). The characteristic color of green liquor is due primarily to the presence of insoluble organic and inorganic sulfides, which are compounds containing sulfur in a chemically reduced state.        c. Causticization (sometimes called re-causticization), is a process where green liquor is combined with, or causticized, with lime (CaO). Causticization regenerates white liquor, along with particulate calcium carbonate, called lime mud. The lime mud is discharged to a landfill or fed to a mud kiln to where it is calcined to re-generate lime for causticization.        
The causticization reactions can be writtenCaO+H2O→Ca(OH)2  (1)Ca(OH)2+Na2CO3→2NaOH+CaCO3  (2)
Note that in Equation (2), sodium is being exchanged with calcium. This is observed in FIG. 1, where the two process loops depicted are labeled the Sodium Loop and the Calcium Loop, and the intersection of the loops occurs in the causticization step. Maximum chemical efficiency and minimal material losses occur when sodium and calcium are contained as completely as possible within their respective loops, with as little exchange as possible of either element (Na or Ca) into the opposing loop. Note also that sodium sulfide (Na2S), which is necessary to the pulp cooking process, does not participate in the causticization reaction, but is simply transferred from green liquor to white liquor as the causticization reaction proceeds.
In conventional kraft pulping, causticization is done by continuously feeding a stream of dry calcium oxide powder into a slaker into which is simultaneously and continuously fed a stream of green liquor. Under such conditions, Reactions 1 and 2 occur in quick succession, beginning in the slaking vessel and continuing over a period of hours as the reaction mixture is fed to a series of causticization tanks. The residence time in the causticization tanks is determined by the flow rates and is set such that the reaction is essentially complete when the mixture exits the last tank in the series. Multiple stirred tanks fed by gravity flow typically are used, and this ensures good mixing of the reactants throughout the process. By carrying out Reactions 1 and 2 via a single-step addition, large, highly agglomerated particles are formed which are easily separated from the white liquor that is co-produced.
Surprisingly, it has been found that by carrying out the reactions defined by Equation 1 and 2 in separate steps, an improved uniformity of size of the lime mud particles is achieved. The degree of agglomeration is correspondingly reduced as well, while maintaining ease of separation of lime mud from white liquor. Even more surprisingly, although the average size of the lime mud particles may be reduced in the process of the current invention (i.e., the particles may, on average, become smaller) the improved particle uniformly and reduced agglomeration result in an increase in the rate of separation of the particles from the white liquor. Along with these benefits, brightness and color of the lime mud particles are improved as well.
The process of the current invention thus produces a brighter, more uniform material that is more suited to applications in which white minerals are typically used. Greater uniformity of particles requires less energy, for example, when milling is used to reduce the size of the particles for those applications that require it.
A kraft causticization process that does not employ a mud kiln is depicted in FIG. 2. In FIG. 2, lime (CaO) that is purchased and stored in a silo is fed to a slaker where it is combined with a stoichiometric excess of filtered green liquor. The mixture from the slaker is fed sequentially to a series of causticization tanks that are filled by gravity overflow. The tanks provide sufficient residence time for the reaction between lime and green liquor to proceed as nearly to completion as possible.
The slurry from the causticization tanks is fed to a filter where the white liquor and lime mud are separated. The time required for filtration and the efficiency of separation are determined largely by the size and size distribution of the lime mud particles. It is generally desirable that the particles be of sufficiently large size to provide fast and substantially complete separation from the white liquor.
Following separation from the white liquor, the concentrated lime mud is fed to a mud wash filter where it is washed with water to remove white liquor that remains trapped in the filter cake. Again, it is generally desirable that the particles are sufficiently large and distributed over a narrow range of sizes so as to minimize the amount of water consumed in washing. The diluted white liquor, or weak wash, recovered during this step is returned to the black liquor recovery boiler where it is used to dissolve smelt to form green liquor.
The washed lime mud can be treated by passing a stream of vaporized carbon dioxide (CO2) through the lime mud to convert any remaining sodium hydroxide (NaOH) to sodium carbonate/sodium bicarbonate (Na2CO3/NaHCO3) thus rendering the pH of the mud sufficiently low as to be safely discharged to landfill. FIGS. 1 and 2 serve to illustrate several operational issues that can reduce the overall efficiency of the recovery process or create problems in disposing of the lime mud, either in a landfill or by calcination in a mud kiln.
First, incomplete separation of sodium compounds can occur when lime mud is separated from white liquor following causticization. Material loss of sodium from the sodium loop represents a cost, as any lost sodium must be replaced. FIG. 1 shows that makeup chemicals are often added to the black liquor recovery boiler where they become part of the smelt.
Also, sodium species that remain with the lime mud must either be removed from the mud via washing or remain with the mud. Kraft processes that employ a mud kiln generally operate to obtain a certain desirable level of sodium in the kiln feed, as this aids in the formation of nodular aggregates that are more efficiently calcined than fine powders. However, too much sodium in the mud leads to the formation of kiln rings that slow or stop the transport of material through the kiln. When they occur, kiln rings must be removed, which requires shutdown and cooling of the kiln and represents a significant operational inefficiency. Undesirably high sodium levels in the mud must therefore be reduced by washing, which can lead to high water consumption, which is both economically and environmentally unsound.
Also, lime mud, which is chemically precipitated during the causticization reaction, is highly suited to trapping and entraining so-called non-process elements (NPEs) which are typically the metal sulfides of elements such as iron (Fe), magnesium (Mg), manganese (Mn) and others. In a kraft process employing a mud kiln, NPEs can build up in the system and must be periodically purged from the system by discharging lime mud in order to maintain process efficiency. Such purging results in an increased amount of lime mud being discharged than would otherwise be required and represents an economic penalty.
Also, lime mud is typically light gray to dark gray in color due to the presence of sulfides. In a kraft process that does not employ a mud kiln, dark lime mud caused by sulfides can result in a diminished ability to utilize the lime mud in applications that are generally suited to the use of conventional calcium carbonate particles. Such applications include paper, paints, plastics, agricultural and other uses. Additionally, lime mud that is intended for use in these applications generally must be milled to a target size and/or specific surface area, which requires energy. Therefore it is desirable that the lime mud particles be formed as aggregates, which aids in filtration, but which are easily broken apart.
It is therefore an object of the invention to improve the separation of white liquor from lime mud by controlling the particle size, size distribution and specific surface area of the mud by controlling the conditions under which the causticization process is carried out in a kraft pulp mill.
It is another object of the invention to optimize the recovery of sodium species in the white liquor while minimizing the amount of water required for washing.
It is another object of the invention to maximize the whiteness and brightness of the lime mud produced from the causticization reaction.
It is another object of the invention to optimize the morphology of the lime mud so that it can be more easily milled to a desired final size, thus requiring less milling energy.
It is another object of the invention to reduce the energy consumed by the operation of a mud kiln in kraft processes that employ such a kiln. This is accomplished by reducing the volume of mud that must be fed to the kiln when the lime mud arising from the invention exhibits improved color and brightness and/or shape, and a larger portion of the lime mud can be used as a pigment in applications where such pigments are typically employed.